Our work during the first award period was focused on defining the functions of HR in the skin and hair follicle. We have been working to understand what controls hairless expression and have shown that hr is regulated by thyroid hormone as well as VDR in keratinocytes. We localized HR is to the nucleus and identified a new nuclear localization signal in the N-terminal region. We also demonstrated that Hr is tightly associated with the nuclear matrix and forms nuclear speckles in association with the histone deacetylase, HDAC3. We showed that HR and VDR are part of the same complex in keratinocytes. In order place a hairless into a transcriptional regulatory pathway, we performed a microarray analysis of rh/rh versus wild-type mouse epidermis to identify genes that are regulated by hairless. Unexpectedly, we uncovered several genes involved in cell cycle regulation and apoptosis, and in particular, numerous genes that are known downstream targets of NFkB signaling. These novel findings form the basis of our experiments planned for the next five years. We will test the hypothesis that hairless is a key regulator of the balance between cell proliferation and differentiation in the epidermis and hair follicle. We will address this hypothesis in three Specific Aims. Specific Aim 1: Transcriptional Regulation of Hairless by the TNF1/NFkB signaling pathway; Specific Aim 2: Regulation of Downstream Pathways by Hairless; and Specific Aim 3: Hairless functions as an inhibitor of proliferation in epithelial cells in vivo. The crucial role of these signaling pathways in epidermal homeostasis and disease places hairless at the center of many convergent areas of skin research. Advances generated by this work could lead to chemopreventive strategies and innovative treatment options for dermatological diseases. PUBLIC HEALTH RELEVANCE: Our work during the first five years of the grant was focused on defining the functions of the hairless gene in the skin and hair follicle. We have found that hairless is a key regulator of epidermal growth, and that it cross-talks with some key signaling pathways in the skin. In the next five years, we will ask how hairless regulates cell growth and proliferation in the epidermis, both in the normal setting, and in certain skin diseases. These findings may prove useful for gaining understanding a number of different skin disorders where uncontrolled growth is a characteristic of the disease.